This study examines associations between social grooming and parasitic infection in wild vervet monkeys (Chlorocebus aethiops) in Loskop Valley, South Africa. Social grooming has been viewed as a behavioral strategy to help hosts rid themselves of parasites (i.e., the hygiene hypothesis). Social contact has been shown to be beneficial to the immune system, however the close contact involved in social grooming may increase parasite transmission. We tested the null hypotheses that there were no differences in proportion of time spent grooming with others and number of grooming partners between groups infected with various gastrointestinal parasites. Focal animal sampling and continuous recording were conducted, and fecal samples were analyzed for gastrointestinal parasites, using fecal flotation, fecal sedimentation, and immunofluorescence microscopy. Individuals infected with Necator sp. had a significantly greater number of grooming partners (F1,36 = 13.18, P < 0.001) and spent significantly more time grooming others (F1,36 = 4.22, P < 0.05) than those not infected. Individuals infected with Entamoeba sp. spent significantly less time receiving grooming from others than those not infected (F1,36 = 10.14, P < 0.004). Close contact involved in social grooming may increase infection with potentially pathogenic parasites, as individuals may ingest infective stages of parasites removed from grooming partners. Individuals who receive less grooming may exhibit increased parasitic infection, through parasites not being groomed off the body, or through reduced immune function from lack of physical contact with others. Thus, the relationship between social grooming and parasites is more complex than the hygiene hypothesis acknowledges.

This study was funded by Wenner-Gren Foundation (#203572), Leakey Foundation (#203294), Purdue Research Foundation, and Purdue University Dept. of Anthropology.